If eBay or buying online is not an option, then your best bet is to get in touch with local collectors or find out if there are some collectible shops in your area that sell used transformers. As long as you don't mind them out of the package. In my area there are at least 3 shops that deal in used Transformers and you'll usually see a pretty good selection of Unicron Trilogy toys in their display cases.
If you find a sizeable group of Transformer fans in the sightings thread for your area they might be able to help you out, both in finding stores and also potentially selling figures. Some collectors decide to sell off entire lines at a time because they no longer want them, and often sell for better prices than you would ever find on eBay.
You could try craigslist too but that's a bit more risky especially because you're still in high school. If you ever decide to meet up with a seller on craigslist make sure it is in a very
public place, never agree to go to someone's home. I've heard horror stories of craigslist transactions gone wrong, so you should always be careful when dealing with people you don't really know. This pretty much applies with anyone you've met online I guess.
Anyway, good luck! It may be a difficult goal to collect the Unicron Trilogy after so much time has passed, but at least it should be fun.